Member Rara Avis
Brad, I don't think there has ever been a time when I pulled a "I make no hypotheses" on anyone. It's that "if you have time" that's usually a problem for me.
Ali, both atmospheric and water pressure are definitely related to gravity. We have a few miles worth of air on top of us, and that's only a little different than having a baby grand piano sitting on top of us. In an open system, pressure is just another word for weight. Of course, that "only a little" difference feels like a pretty big difference if you happen to be the one stuck under that baby grand.
The difference is that air and water "flow" and we feel their "weight" from many different directions. Okay, it's a pretty big difference, I guess. Because we would, otherwise, be crushed. A ten by ten inch square of paper has about 1,450 pounds of air sitting on top it. Ouch. But because air "flows," it also has 1,450 pounds of air pushing in the other direction.
It works exactly the same way with water, except the weights are even more staggering. At the bottom of the Mariana Trench the pressure is over 8 tons per square inch. That same piece of paper would now have 8,000,000 TONS sitting on it. However, a steel girder at the bottom of Mariana Trench would NOT be crushed by that weight, any more than the rocks down there are. The water - and the water pressure - flow all around the girder and everything is cool. A closed box made out of similar girders would be instantly crushed, though, because the water couldn't "flow" inside it to counteract the pressures on the outside (well, it can and it would - explosively so).
(Related Aside: Boyoncy in water has very little to do with lung capacity. Put two quarts of air in a steel can weighing 150 pounds, or four quarts of air, and the can is still going to sink. I had a friend in high school who could sit on the bottom of the pool for two minutes. He didn't float. The reason he didn't float was that he remarkably lean, and muscle has a higher density than water. Fat, on the other hand, floats. )
You are essentially right about the effects of gravity when you say the higher you are the less gravity affects you. Interestingly, though, it's not quite that simple.
Gravity, of course, is described as nothing more than the attraction between two objects and depends only on the mass of the objects and their distance from each other. Note, it's a two-way attraction. Yes, the gravity of Earth is attracting us, pulling us towards the center of the Earth. But we're attracting the Earth, too. It's just so small as to be immeasurable. That's because the strength of the attraction is directly proportional to the amount of mass. For example, we "weigh" less on the moon than we do on the Earth just because the moon has less mass than the Earth.
Here's something cool. Spherically symmetric objects (planets, stars, moons, etc.) behave as if all of their mass is concentrated at their centers. Black holes are often called singularities, because all their mass has collapsed to a single point. In a sense, all the planets and stars are singularities, not because their mass has collapsed, but because the entire force of their gravity is concentrated in a single, dimensionless point.
The strength of the attraction between two objects depends on mass and, as you surmised, Ali, on distance. It's called the Inverse Square Law and simply means that gravity decreases in proportion to the square of the distance. Right now, you're on the surface of Earth, roughly 8,000 miles from its epicenter. Let's say you weigh 200 pounds. If you move another 8,000 miles from the epicenter, you are twice as far away. But you don't weigh half as much. Gravity is determined by the square, two squared is four, so you weigh one fourth as much, or just 50 pounds. Go another 8,000 miles. You're now three times as far from the epicenter, three squared equals nine, so you weigh one ninth as much, or about 22 pounds.
The Inverse Square Law means the strength of gravity drops off VERY quickly as the distance increases. But it never quite drops off to zero. The gravity from our insignificant planet has an affect, however tenuous and immeasurable, on every star in every galaxy. Everything in the Universe really is connected to everything else.
Weightlessness? There is no such thing. What we see as weightlessness when the space shuttle orbits the Earth is a very carefully calculated flight path. If the Earth's gravity didn't extend farther than the shuttle orbit, after all, the moon would have been whipped into space a long time ago. What really happens is the speed and centrifugal force of the orbiting shuttle exactly cancel out the gravity, giving an illusion of being weightless.
And that raises what I think is one of the most interesting things about gravity. If you were in a closed box and suddenly felt yourself "pulled" toward the floor of the box, there is no way to know whether that "force" is the result of gravity or acceleration. We all know what it feels like when a fast elevator starts moving? It's the same feeling you'd experience if the mass of the Earth suddenly increased. Gravity and acceleration are indistinguishable.
In a way, it makes sense. If you jump out of a plane at 5,000 feet, it is gravity that propels you towards the ground. Faster and faster. Falling is nothing more than accelerating, albeit in a usually undesired direction.
Gravity is considered one of the four fundamental forces I mentioned in my last post, along with the electromagnetic force, the strong-nuclear force, and the weak-nuclear force. The concept of "force" and much of modern physics goes back all the way to Aristotle. He invented an invisible and frictionless medium called "ether" to explain how things like gravity and magnetism could apparently cause "action at a distance." We finally gave up the idea of ether (somewhere around 1881, I think, when Michelson and Morrley were measuring the speed of light and setting Einstein up to answer the questions they unearthed), but science is still very much determined that cause and effect cannot occur across distance. That is, in fact, a fundamental premise of modern science. The actions at a distance are, today, believed to be caused by particles. We have photons and electrons, for example, to describe the electromagnetic force, and hadrons account for the strong nuclear force. Those I've mentioned have been detected and the theories are hard to debate.
The graviton, however, which is the particle that accounts for gravity's ability to act at a distance, has never been detected. In some ways, that's not surprising, just because it's by far the weakest of the four forces. The strong-nuclear force (holds together the nucleus of atoms) is one hundred times stronger than the electromagnetic force, one million times stronger than the weak force, and 10 to the 38th power stronger than gravity. Gravity is a major weenie, and that makes it hard to find the theorized particle. (Gravity is only strong, by far the strongest of the four forces, in the sense of accumulation. All the other forces are limited by their own characteristics. The weak force works only at distances smaller than the size of an atom, and the positive/negative features of the electromagnetic force tend to cancel each other out. Gravity seems to have no such limitations. You want more gravity, you just add more mass.)
In theory, everything emits gravitons. When the gravitons meet, they interact to create the "attraction" we experience, not unlike the way we know electrons create the attraction and repulsion we see in magnetism. The trouble is, electrons are very easy to detect and prove. Electrons are strong. Gravitons are so incredibly much weaker that we still haven't been able to find one.
As I said before, I personally don't think we ever will. I think we've incorrectly grouped gravity with the other three forces, putting it there simply because it appears to work "at a distance" in the same way the others do. But what if gravity is actually more fundamental than the fundamental forces?
I don't have any real answers, of course, just a gut feeling based on things I've read. But gravity and motion seem to have much more in common with each other than gravity does with any of the other forces. No, I'm not suggesting an integral connection. Motion, rather, is an aspect of space. I think we're going to find that gravity, rather than being grouped with the forces, will eventually find itself on par with time and space, the true fundamentals. I suspect it will be the link between what we call space and what we call matter/energy.
Thus go the ramblings of true amateur.